[Wasp_Factory]

You want 20" per the first post or 21" per the second? Not seen anyone fit 21" to the G42.

Assuming you want 20", the first question is whether you want to go wider than the M Tech wheel option which is 245/35/19 front and 255/35/19 rear. I have this set up and it is plenty of tyre on a 240iX on the road.

If you want to go for 20", at these widths, you'd simply need to drop to 245/30/20 front and 255/35/20 rear. The Michelin MPS4S star would be a decent choice and keeps the car close to OEM spec.

For offset, I have aftermarket wheels and went with ET 20 at the front and ET 30 at the rear on Eibach lowering springs (-20mm f&r). I don't have the fender flares. I feel you could go to ET15 front and ET20 rear giving you an extra 5mm poke (fender flares are 4mm I think) and you'd have the perfect stance with as much poke as you can legally get away with. You need 66.6mm bore, otherwise you'll need spigot rings.

If you want to go wider, take a look at Motech Performance in the UK - they've gone wider at the rear on 20".

The above will keep you within the 1% rolling radius delta front to back but probably means that you will need to go the custom alloy route to get exactly the size, offset, colour, centre bore and bolt pattern that you need. This will allow you to use the centre caps off your OEM wheels as well.

Hope this helps.